Atomic clocks get a grip on gravity

MEASURING Earth's hidden structures could soon be as simple as looking at your watch - provided it's a super-accurate atomic clock.

Such clocks are nearly good enough to deliver a detailed geoid, says Ruxandra Bondarescu at the University of Zurich, Switzerland, and her colleagues. The geoid is a model of Earth's density variations - from the surface down to the mantle - as revealed by anomalies in the planet's gravitational pull. Knowing the geoid's shape can aid studies of deeply buried geological structures and show how mass is being redistributed over time, such as by the melting of polar ice sheets.

Geoid measurements use satellite readings of Earth's gravitational field, but they are limited to a resolution of about 400 kilometres.

General relativity tells us that clocks run slightly faster above sea level due to lower gravity and slower below due to higher gravity, so an atomic clock carried around the surface could also measure the geoid, Bondarescu says. The team's calculations show that large clocks in labs are already accurate enough to determine geoid heights to the nearest centimetre (arxiv.org/abs/1209.2889).

The clocks still need to be more portable, but Roger Haagmans at the European Space Agency says miniaturised versions are feasible.

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